I think you actually pull it out not push in, half way out is front fogs and all the way out is rear fogs as well. That's how my old mk2.5 was. It doesn't actually turn to the position shown, it just goes on how far out the switches pulled.

Next problem - there was no bulb so thought I was home and dry but single centre and side contact bulb same as reversing does not work, and the interior of the holder seems unduly complicated so I have no idea of the physical characteristics of the bulb I need.

Looking on the halfords website it definitely shows the rear fog and reversing light are the same, try putting the reversing light in to see if it works. The fact the bulb is missing may suggest there is a fault and someone took out to stop it blowing etc

theres your problem stan you need a new holder as that one is corroded to hell will never work and will be the cause of blown fuse if that has gone as well i will have a look in my array of spares as i,ve changed lights on other focus so possible got a spare holder you can have will get back to you if i can find one even throw a bulb in